Was Sisera murdered in his sleep?

Yes, he was fast asleep.

Then Jael Heber's wife took a nail of the tent, and took an hammer in her hand, and went softly unto him,
and smote the nail into his temples, and fastened it into the ground: for he was fast asleep and weary. So
he died. Judges 4:21

No, he was awake and standing up.

She put her hand to the nail ... and with the hammer she smote Sisera.... At her feet he bowed, he fell,
he lay down: at her feet he bowed, he fell: where he bowed, there he fell down dead. Judges 5:26-27